Linear motors are described in Hazelton et al. U.S. patent application Ser. No. 09/059,056, filed Apr. 10, 1998, the specification of which is incorporated herein by reference in its entirety, and which is assigned to Nikon Corporation, Assignee of the present application. Linear motors are commonly used, for example, in micro-lithographic instruments for positioning objects such as stages, and in other precision motion devices. A conventional linear motor includes a magnet array, which creates a magnetic field. A linear motor generates electromagnetic force (traditionally called Lorentz force) on a moving linear coil array in cooperation with a stationary magnet array to propel a moving stage attached to the coil array. Alternatively, a linear motor can have a stationary coil array and a moving magnet array attached to a stage.
FIG. 1a is an isometric view of a typical magnet array 100 currently used in linear motors. Magnet array 100 includes a base rail 102 and two side rails, e.g. a right side rail 104 and a left side rail 106, which are attached to base rail 102 by screws 108 to form a "U" shaped yoke 114 containing an open channel 116. Full size magnets 110 and short magnets 112 are attached to side rails 104 and 106 to provide magnet pairs, which face each other across channel 116. Short magnets 112 are positioned adjacent the open ends of yoke 114 and full size magnets 110 are typically equally spaced along each side rail 104, 106 between respective short magnets 112. The magnets of each magnet pair have parallel magnetic polarities. Adjacent magnet pairs have oppositely directed magnetic polarities.
Base rail 102 is of non-magnetic material, such as 304 stainless steel, aluminum or ceramic. Side rails 104 and 106 are of magnetic material (e.g. iron or steel) typically with saturation flux density equal to or greater than about 16,000 gauss. Full size magnets 110 and short magnets 112 are of e.g. high quality neodymium iron boron (NdFeB) permanent magnet material with a residual permanent magnetic flux density of about 13,500 gauss or greater. The magnets are typically coated to prevent corrosion.
Conventionally, a linear motor includes a coil array (not shown), located in channel 116. The coil array generates an electromagnetic force in cooperation with magnet array 100.
Such a conventional linear motor has several disadvantages, one of the which is low efficiency. Additionally, massive side rails of iron or other magnetic material are required to complete the magnetic flux circuit. These side rails can add undesired weight to a moving magnet array.
Trumper et al., "Magnet Arrays for Synchronous Machines," Proc. of IEEE Industry Application Society Annual Meeting Oct. 2-8, 1993 (IEEE CAT93CH3366, vol. 1 of 3 vols.) describes a Cartesian Halbach magnet array design (see also Halbach "Design of Permanent Multipole Magnets with Oriented Rare Earth Cobalt Material," Nuclear Instruments and Methods, 169, 1980, pp. 1-10; "Physical and Optical Properties of Rare Earth Cobalt Magnets," Nuclear Instruments and Methods, 187, 1981, pp. 109-117).
FIG. 1b is a schematic view of a Cartesian Halbach array 150, as described by Trumper et al., cited above. Halbach array 150 contains a row of abutting permanent magnets 152a-152g of alternating orthogonal polarity, as shown by arrows in each block of FIG. 1b. Halbach array 150 has a spatial period D152 equal to the widths of four magnets 152a-152d, in which the magnetic polarity rotates stepwise by 90 degrees per magnet through a full angle of 360 degrees. For example, magnets 152a and 152e (which is one spatial period D152 or four magnet widths displaced from magnet 152a) have the same magnetic polarity. The thickness D150 of Halbach array 150 is equal to one quarter of spatial period D152.
The magnetic flux circuit of Halbach array 150 is completed through the magnets 152a-152g, such that no iron or other magnetic material is required for side rails. Magnetic flux is shown by the solid curves in FIG. 1b. It is evident from FIG. 1b that the magnetic flux is substantially higher on a strong side 154 and lower on a weak side 156 of Halbach magnet array 150. When a linear motor is configured with paired Halbach magnet arrays having strong sides 154 facing one another across a channel, then the magnetic flux at the intermediate coil array is advantageously enhanced. Thus the highly asymmetric concentration of magnetic flux in Halbach magnet array 150 provides an improvement in linear motor efficiency relative to conventional magnet array designs, such as that shown in FIG. 1a.
Nevertheless, further efficiency improvements have been sought during more than a decade since the advent of the Halbach design. Clearly needed in the art is a magnet array that provides higher magnetic flux density for a given width of magnet array, relative to the Halbach design. Also needed in the art is a magnet array that provides magnetic flux enhancement without requiring a magnetic side rail, and can therefore use a side rail material with high stiffness and low mass, such as ceramic.
It is accordingly desirable to provide a magnet array for a linear motor having enhanced magnetic flux in a compact configuration, and having flexibility in the selection of side rail materials.